Transplantation of stem cells is accepted as a way to boost tissue regeneration in preclinical trials, and this strategy is being translated to the bedside (Kolonin and Simmons, Nat. Biotechnol. 27, 252-253, 2009). Stem cell therapy would greatly benefit from a capacity to target mesenchymal stromal cells (MSC) in specific organs for imaging or therapeutic applications. MSC are a mixed cell population that comprises multipotent adult progenitor cells (Bianco, et al., Cell Stem Cell 2, 313-319, 2008). Due to the ability of these cells to differentiate into mesenchymal lineages, such as osteoblasts, chondrocytes, and adipocytes, they are commonly referred to as mesenchymal stem cells (Pittenger, et al., Science 284, 143-147, 1999; Prockop, Science 276, 71-74, 1997). Recent studies reveal that MSC function as perivascular cells (pericytes) maintaining vascular integrity (Crisan, et al., Cell Stem Cell 3, 301-313, 2008; Tang, et al., Science 322, 583-586, 2008; Traktuev et al., Circ. Res. 102, 77-85, 2008).
Preclinical studies and clinical trials with transplanted MSC support the therapeutic potential of these cells and suggest that these cells are also activated during disease to participate in tissue repair and regeneration. However, the clinical application of MSC for cell therapy require the ability to track and control this plastic cell population, due to safety concerns related to their capacity to promote cancer (Zhang et al., Cancer Res. 69, 5259-5266, 2009).
MSC were originally isolated from the bone marrow stroma and termed fibroblast colony-forming units (CFU-F) based on their fibroblastic morphology (Friedenstein, Haematol. Blood. Transfus. 25, 19-29, 1980). Since then, MSC have been identified in the majority of adult organs, with white adipose tissue (WAT) being the largest reservoir of MSC, these cells being termed adipose stromal cells (ASC: Gimble et al., Circ. Res. 100, 1249-1260, 2007; Daquinag et al., Trends in Pharmacol. Sci. 22, 1-8, 2011). While WAT is primarily composed of adipocytes, ASC constitute the majority of cells in the stromal/vascular fraction (SFV), which also contains endothelial cells (EC) and infiltrating hematopoietic cells (Hausman et al., Obes. Rev. 2, 239-254, 2001). ASC have been revealed to be a rich source of multipotent progenitors that display multipotency and proliferation capacity comparable to those of bone marrow MSC, while also having clear unique features (Rodeheffer et al., Cell 135, 240-249, 2008; Tang et al., 2008, ibid; Zuk et al., 2001, Tissue Eng. 7, 211-228).
Current understanding of the mechanisms through which intercellular interactions of MSC with other tissue components mediate tissue remodeling is limited by the lack of specific MSC markers. While a number of cell surface molecules, including platelet-derived growth factor receptor-β (PDGFRβ), CD146, CD44, CD90, CD105, CD73, CD29, and Stro-1, have been used for positive selection of MSC, each one of them is also expressed on other cell types (Bianco et al., 2008, ibid; Gimble et al., 2007, ibid; Nie et al., Stem Cells 26, 2735-2745, 2008). MSC (including those from WAT) can be distinguished from hematopoietic cells based on the lack of the pan-leukocyte marker CD45 and from endothelial cells (EC) based on the lack of the pan-endothelial marker CD31/PECAM-1 (Bianco et al., 2008, ibid; Rodeheffer et al., 2008, ibid). Because CD34 is expressed on perivascular MSC of several organs in vivo, the CD34+CD45−CD31− immunophenotype has been used to separate MSC from other cells (Gimble et al., 2007 ibid; Tang et al., 2008, ibid; Traktuev et al., 2008, ibid; Zhang et al., 2009, ibid). However, because this signature does not differentiate MSC in different tissues, new markers for prospective isolation and tracking of ASC and MSC in other tissues are urgently needed and are constraining the field of stem cell transplantation as a whole.
Furthermore, adipose associated disorders include, but are not limited to, obesity, body weight and body composition disorders, including eating and other disorders effecting regulation of body fat and body weight, represent major health problems in all industrialized countries. The prevalence of obesity in developed countries is high. For example, by the year 2002 approximately 30% of Americans were obese (National Health and Nutrition Examination Survey). Obesity is a condition in which there is an excess of body fat relative to lean body mass, and is defined as a body mass index (weight/height2) greater than 30 kg/m2 (Kopelman, Nature 404:635-643, 2000). Obesity is associated with an increased risk of developing many serious diseases, including, but not limited to, type 2 diabetes, stroke, hypertension, coronary artery disease, certain cancers, chronic fibrosis in certain tissues, fatty liver disease, chronic venous insufficiency, deep vein thrombosis, arthritis, breathing problems such as obstructive sleep apnea, and gallbladder diseases such as cholelithiasis, among other complications (Willett et al., N. Engl. J. Med. 341:427-434, 1999).
Of further note, stromal cells are connective tissue cells that support the function of the parenchymal cells of an organ. Fibrosis is a pathologic process, which includes scar formation and over production of extracellular matrix, by the connective tissue, as a response to tissue damage. Confluent fibrosis can obliterate the normal architecture and function of the underlying organ or tissue. Fibrotic disorders, include but are not limited to, pathologic scarring in the skin, such as, colloid and hypertrophic scar; cirrhosis of liver and gallbladder, fibrosis in the heart, eye and the kidneys, pulmonary and bone-marrow, fibrosis in the gastro intestinal tract, that associated with cancers, such as sarcomas and hemangiopericytoma as well as scleroderma. Additional fibrotic disorder include scurvy and autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus and genetic disorders examples of which include Marfan syndrome, Ehlers-Danlos syndrome, Loeys-Dietz syndrome, pseudoxanthoma elasticum, osteogenesis imperfect, fibrodysplasia ossificans progressive and spontaneous pneumothorax, among others.
The above mentioned disease associations explain why the economic costs of obesity are estimated to be more than 100 billion dollars (Daniels, Am. J. Nurs. 106:40-49, 2006), which is estimated to be as much as 7% of total health care costs (Seidell, Int. J. Obes. Relat. Metab. Disord. 19(Suppl 6):S13-S16, 1995).
The regulation of body composition in mammals is a complex process that involves the regulation of both appetite and energy expenditure. Human body composition and body weight homeostasis is poorly understood. The epidemiology of obesity strongly shows that the disorder exhibits inherited characteristics, and human twin studies strongly suggest a substantial genetic basis in the control of body composition and body weight, with estimates of heritability on the order of 80-90 percent. Studies of the genetics of human obesity and of animal models demonstrate that obesity results from the complex interactions of defective regulation of food intake, food induced energy expenditure, and the balance between lipid and lean body anabolism. Thus, obesity is not merely the result of negative behavior, i.e., the result of voluntary hyperphagia, but results from differences in eating patterns, metabolism, and neurologic/metabolic interactions. These differences seem to be due, in some degree, to differences in gene expression, either the level or activity of gene products (Friedman et al., Mamm. Genome 1:130-144, 1991).
Current methods for control of body weight include dieting and surgical procedures. However, diet is often unsuccessful and the few obesity therapeutics approved by the U.S. Food and Drug Administration, such as Phentermen, fenfluramine, Meridia, Xernical, Orlistat, Adipex-P, Bontril and Ionomin have unacceptable or dangerous adverse effects and those few that have been approved have been removed from the marketplace. Surgical methods for weight reduction, such as liposuction, gastric bypass or banding surgeries, have many risks. None of the presently available methods for weight control is satisfactory and a need exists for improved methods of weight loss and control.
In addition to the obesity problem in the human population, there is a growing problem of obesity in companion animals, such as dogs and cats (German, J. Nutr. 136:1940S-1946S, 2006). In the United States, it is estimated that 25-40% of all dogs and cats are overweight or obese. As in humans, obesity can have detrimental effects on the health and longevity of companion animals. Additionally, the ability to reduce or eliminate the use of body composition and body weight manipulators in commercial livestock feed has significant utility not only in savings to the grower, but to the consumer and to society as a whole. Therefore, the ability to manipulate body composition and body weight and to treat body composition disorders in humans and companion animals such as, but not limited to, obesity, and the ability to increase the size of livestock, have broad utility and represent significant opportunities.
Diabetes is a long-term, currently incurable disorder, associated with body composition disorders, specifically obesity and the metabolic syndrome, with greatly increased risks for developing additional pathologic conditions resulting from poor glycemic control. Chronic, short-term risks include, but are not limited to, hypoglycemia, infections, and disorders associated with hyperglycemia, such as ketoacidosis. Long-term complications resulting from diabetes include, but are not limited to, heart disease, stroke, high blood pressure, vascular disease, visual impairment, nephropathy, and neuropathy. According to a 2005 survey by the National Institute of Diabetes and Digestive and Kidney Diseases, over 20 million people in the United States have diabetes, and the percentage of the population with diabetes is rapidly increasing. In 2005 in the United States, 1.5 million people were diagnosed with diabetes. Some studies project that about 250 million people worldwide will be afflicted by diabetes by the year 2020 (O'Rahilly, BMJ, 314: 955-959, 1997). The most prevalent forms of this disorder are insulin-dependent diabetes mellitus (IDDM or type I) and non-insulin-dependent diabetes mellitus (NIDDM or type II). Due to the high incidence of diabetes, and the irreversible damage that is incurred with many of the associated complications, the cost of treatment surpasses any other single disease in the United States. In 2002, more than $132 billion was spent on direct and indirect costs for treatment, with about $92 billion of that going to direct medical costs (Hogan et al., Diabetes Care 26:917-932, 2003).
Recently, obesity has emerged as one of the factors predisposing cancer patients to mortality as a result of advanced cancer progression (see for example, Daquinag et al., Vascular targeting of adipose tissue as an anti-obesity approach. Trends in Pharmacol. Sci. 22, 1-8, 2011; Zhang, et al. Adipose-tissue derived progenitor cells and cancer, World Journal of Stem Cells (Topic Highlight), 2 (5): 103-113, 2010; Bellows, et al., Influence of BMI on Level of Circulating Progenitor Cells, Obesity, online; Flegal, et al. Cause-specific excess deaths associated with underweight, overweight, and obesity. JAMA. 298, 2028-2037, 2007; Roberts, et al. Biological mechanisms linking obesity and cancer risk: new perspectives. Annu Rev Med. 61:301-316, 2010). Survival of patients with prostate and breast carcinomasis decreased by obesity more than for other cancers. WAT has a direct effect on cancer progression (Vona-Davis, et al., Adiposity, type 2 diabetes and the metabolic syndrome in breast cancer. Obes Rev; 8: 395-408, 2007) as a potent endocrine organ secreting numerous soluble adipokines implicated in inflammation and the metabolic syndrome (Rosen et al., Adipocytes as regulators of energy balance and glucose homeostasis. Nature; 444: 847-853, 2006). Insulin-like growth factors (IGFs) are among those that could directly stimulate tumor cell proliferation. Leptin, interleukin-6, as well as various other hormones and inflammatory cytokines, may also play a role (Baillargeon. Obestity adipokines and prostate cancer (review). Int J Oncol; 28: 737-745 2006). However, clinical and experimental data addressing the role of these factors in cancer has remained controversial. For example, in animal models ablation of circulating IGF-1 has shown no effect on prostate tumor growth (Anzo et al., Targeted deletion of hepatic Igf1 in TRAMP mice leads to dramatic alterations in the circulating insulin-like growth factor axis but does not reduce tumor progression. Cancer Res; 68(9):3342-9, 2008). Therefore, alternative mechanisms must be considered.
As opposed to WAT, whose physiological function is to store excess energy, brown adipose tissue (BAT) is responsible for energy dissipation in the form of heat. The fine balance between WAT and BAT is an important issue to take into consideration in designing targeting therapies. A number of results from the rodent models indicate that BAT has a protective effect against the pathological consequences of obesity. The significance of discovering BAT in adults lies in possible new approaches to treatment of obesity and of the associated disorders. ASCs are one of the progenitor cell types giving rise to BAT (Elabd et al. Human multipotent adipose-derived stem cells differentiate into functional brown adipocytes. Stem Cells 27, 2753-2760, 2009). More importantly, white adipocytes can be directly converted to a BAT-like phenotype both in culture and in vivo (Cinti. Transdifferentiation properties of adipocytes in the adipose organ. Am. J. Physiol. Endocrinol. Metab. 297, 977-986, 2009). In mice, expansion of residual BAT “patches” within WAT can lead to virtually all adipose depots becoming BAT-like at the expense of WAT. The apparent dependence of adipocyte physiology on the status of the vasculature suggests that vascular-targeting agents could be designed to convert WAT into BAT, rather than destroying tissue altogether, as a more physiological and safer anti-obesity treatment. Development of pharmacological approaches to activate proliferation, vascularization and/or metabolism of the existing residual BAT could, in theory, tilt the WAT/BAT balance and be used to treat obesity.
In summary, adipose associated body composition and body weight disorders such as obesity and diabetes represent major health problems. Given the severity, prevalence, and complexity of adipose associated body weight disorders, there exists a great need to identify genes and proteins that participate in body composition and body weight control, and to develop new drugs and therapies to treat adipose associated body composition disorders, obesity, metabolic syndrome, and diabetes. Targeting treatment and imaging modalities to different type of cells in adipose tissue (WAT and BAT), such as ASC, can be beneficial for biomedical applications in obesity, cancer and fibrosis.